Deaerator system and method for deaeration

ABSTRACT

In a method for deaerating a liquid the liquid is pressurized to a pressure above atmospheric, after which it is guided to an upstream end of a nucleation valve. A low pressure resides on the downstream end of the nucleation valve and as the liquid passes the valve, bubble nucleation is initiated, forming the first step in a deaeration process. According to the method the temperature and pressure on the downstream side of the valve is controlled such that the static pressure is above the saturation pressure, while the lowest pressure as the liquid passes the valve is below or equal to the saturation pressure.

TECHNICAL FIELD

The present invention relates to a system and a method for deaeration ofa liquid food product.

BACKGROUND

Within the field of packaging of liquid products deaeration is a wellestablished concept and deaeration is included as an essential step inmost processing plants e.g. in plants where liquid product is receivedas a bulk in a first end of the line and delivered as individualpackaging containers in the other end of the line. Air (or oxygen) maybe present in the liquid product for different reasons, the two mainreasons being that it is dispersed or dissolved. Taking the example ofmilk, there will be some oxygen in the milk already before it has leftthe cow, and more oxygen will be mixed and dissolve into the milk ateach processing step starting with the milking process itself.

The air and oxygen may result in several negative effects ranging fromreduced skimming efficiency in separators, cavitation in the productduring processing, fouling of heating surfaces in pasteurizers,shortened product shelf life (due to oxidation), etc., which are somereasons to why deaeration is a well-established processing step.

To simplify the underlying theory, which obviously is well- establishedand well-known to the skilled person, the solubility of a gas such asoxygen or nitrogen in a liquid will depend of temperature and pressure.At lower temperatures more oxygen or nitrogen may be dissolved in theliquid than what is the case at a higher temperature, i.e. thesaturation concentration is higher at a lower temperature. For pressurethe relationship is reversed, the higher the pressure the higher thesaturation concentration. This simple relationship establishes that inorder to deaerate a liquid one or both of the temperature or thepressure may be altered. Also, it may be obvious that deaeration as suchis not difficult to accomplish by simply dialing in the desiredtemperature and pressure of a particular saturation concentration in avessel containing the liquid. In a commercial production line, however,the deaeration should allow treatment of thousands of liters of liquidproduct per hour with a requirement of being energy efficient whichrenders the theoretical approach of awaiting equilibrium to be reachedinapplicable. Still, there are deaeration techniques used which arequite similar to the theoretical approach.

A deaeration method more commonly used in the main field of the presentinvention is to make use of a vacuum deaeration in an expansion vesselconnected to vacuum. The liquid is transported to the expansion vesselwith a certain temperature which is some degrees above the boiling pointat the pressure prevailing in the expansion vessel. When the liquidenters the vessel via a valve and the temperature and pressureconditions in the vessel causes it to instantly start boiling, a processreferred to as flash boiling (flash or flashing in the following). Theprocess results in that liquid in vaporized and that air is releasedfrom dissolved form during flashing. Liquid vapor condense againstcooled areas in the vessel, while the released air is evacuated from thevessel by the vacuum pump, while the liquid exits through an opening inthe bottom of the vessel. In order to increase the separation rate theliquid may enter the expansion vessel in a tangential direction, so asto induce a swirl. This deaeration method is very efficient, yet intimes of increasing energy costs as well as increased energy awarenessthere may still be room for improvements.

SUMMARY

For obvious reasons there are similarities between the present inventionand the prior art in terms of the result to be achieved. However, afundamental difference is that while the background art focuses onaffecting the conditions at some location after the valve, most commonlythe pressure and temperature in the expansion vessel, the presentinvention focuses on affecting how the fluid transitions from theconditions upstream the valve to the conditions downstream the valve,and on processing of the liquid prior to it reaching the separationvessel. Some parameters to adjust are the pressure upstream the valveand the pressure downstream the valve. In this way the pressure dropover the valve may be controlled. The dimensions of the restriction inthe valve will in turn affect the flow velocity through the valve andthus the transition time. Subjecting the fluid to an instantaneous andsignificant pressure drop will induce nucleation. Experiments revealthat the nucleation of (gas) bubbles occurs in the entire volume of thefluid, i.e. a homogenous nucleation, and that it therefore facilitatesan efficient deaeration. For one or more embodiments of the presentinvention it is preferred that even if the equilibrium pressure andtemperature downstream the valve are such that the fluid does not flash,the minimum pressure caused by the restriction still will inducecavitation of the fluid, which also will facilitates deaeration.

To this end the present invention relates to a method for deaerating aliquid, comprising the steps of pressurizing the liquid to a pressureabove atmospheric, guiding the pressurized liquid to an upstream end ofa nucleation valve, and lowering the pressure on a downstream side ofthe valve to a subatmospheric pressure, thereby causing gas bubblenucleation as liquid passes the nucleation valve as the first step ofthe deaeration. Relying on the gas bubble nucleation as a first step inthe deaeration process differs from prior art technique where flashingis the predominant effect utilized and the inventive method provides anenergy efficient and time efficient deaeration method.

In one or more embodiments the method comprises the step of forming afree fluid jet with the nucleation valve. Experimental results indicatethat the formation of a high-velocity jet flow, which in mostapplications will be a turbulent jet flow, will enhance the deaeration.The free jet flow is preferably not restricted (e.g. directed into awall). In this context it should be noted that the flow is contained insome sort of piping or similar, and that some part of the formed jetwill interact to some degree with the walls of the piping. The core ofthe jet will however not interact to any significant degree with aconstructional delimitation of the system.

The method may also comprise the step of inducing a pressure drop overthe valve, the pressure drop preferably exceeding 2 bar, more preferablyexceeding 3 bar, suggestively being around 4 bar or 5 bar. Experimentsindicate that an increased pressure drop results in an increaseddeaeration efficiency. It is indeed possible to apply higher pressuresupstream the valve (in order to increase the pressure drop) yet thereare practical constraints, e.g. in terms of pump capacity.

According to one or more embodiments thereof the method comprises thestep of controlling the pressure downstream the valve such that itremains above or at the saturation pressure of the liquid. This willeliminate the risk of flash boiling on a larger scale.

In one or more embodiments the method comprises the step of guiding theliquid leaving the nucleation valve downstream into a diffusion reactor.In the diffusion reactor, into which the free jet is directed, dissolvedgas in the liquid will diffuse from the liquid into the gas bubbles.

In order to separate the gas from the liquid further the presentinventive method, in one embodiment thereof, may comprise the step ofguiding the liquid leaving the diffusion reactor downstream into aseparation vessel. This is preferably performed by having the diffusionreactor debouching directly into the separation vessel, in which gasphase is separated from liquid phase.

It is believed that the sudden pressure drop as such may be an importantfeature, yet it also seem beneficial to, in one or more embodiments ofthe present invention, control the pressure on the downstream side ofthe valve such that it is lower than 0.1 bar. In more general terms itmay be stipulated that the pressure immediately after the valve shouldremain below 160%, such as below 150%, of the saturation pressure forthe liquid at the particular temperature.

In one or more embodiments the pressure on the downstream side of thevalve is controlled such that the static pressure is above thesaturation pressure, while the lowest pressure as the liquid passes thevalve is below the saturation pressure. As the liquid passes the valveit will be accelerated to a high-velocity flow, resulting in localpressure reduction due to the dynamic pressure. If the ambient staticpressure is close to (above or at) the saturation pressure, the dynamicpressure may cause a drop below the saturation pressure. This will causelocal flash boiling, which is believed to facilitate deaeration further.

According to another aspect of the present invention it relates to asystem for deaerating a liquid according to the inventive method andembodiments thereof. An inventive system for deaerating a liquid,comprises a pump for increasing the pressure in the liquid on anupstream end of a nucleation valve, a vacuum pump for reducing thepressure on a downstream end of the pressure reducing valve and acontrol system for controlling the pumps.

According to one or more embodiments the nucleation valve provides anunrestricted flow after the pressure drop, such that a free fluid jetmay be formed. Some valves have an intricate construction where the flowmust follow an intricate channel after the main pressure reduction inthe valve. Experiments indicate that such valves are less suitable foruse in the present invention, and that the valve preferably should beoff a non-complex design, at least following the main pressure drop,such that a free jet flow of liquid may extend from it during operation.

The control system may be adapted to induce a pressure drop over thevalve preferably exceeding 2 bar, more preferably exceeding 3 bar,suggestively being around 4 bar or 5 bar. In one or more embodiments thecontrol system is adapted to control the temperature and pressuredownstream the valve such that the liquid is kept below or at itsboiling point.

An inventive system, according to one or more embodiments thereof mayalso comprise a diffusion reactor arranged downstream the nucleationvalve and in some embodiments also a separation vessel arrangeddownstream the diffusion reactor.

In embodiments where a diffusion reactor is used it is preferred that ithas an elongate shape, and that it is rectilinear to minimizeinterference with the jet and the flow thereafter. The diffusion reactormay in one or more embodiments have a length exceeding about 100 cm,preferably exceeding 150 cm and more preferably a length of about 200cm, and the width may in these or other embodiments be about 4-10 cm,preferably about 5 cm, for a circular cross section. The measures areexemplifying only, and mainly relevant for a flow in the order of1-10,000 I/h. For higher flows, which are common within the field of thepresent invention the size of the diffusion reactor should preferably bescaled up. It is then a general preference to increase the cross sectionrather than the length of the diffusion reactor, and the cross sectiondimensions will scale linearly to the flow, such that a doubled flowleads to a doubled cross section.

In one or more embodiments at least 50% of the mass transfer fromdissolved phase to gaseous phase occurs in the diffusion reactor, and inseveral embodiments significantly more than that. This measure will relyon the dimensions of the diffusion reactor, yet to a high degree it willdepend on the operation parameters (pressure, temperature, flow rate)too. Even if the parameters may be quite complex, a simple measurementmay be used to verify that the criterion is fulfilled. The feature assuch is significantly different from most commercial systems where thetransfer takes place in the expansion vessel.

According to another aspect of the present invention there is provided adiffusion reactor as defined above and below, which may preferably beused in the previously described inventive method and mounted downstreamthe described nucleation valve, or may be added as an additionalcomponent between a pressure reduction valve and the separation vesselof a conventional system. The purpose of the diffusion reactor is toallow for the gas molecules (i.e. dissolved gas) to further diffuse fromthe liquid to the now existing and growing nuclei/gas bubbles. A methodof deaeration including the step of guiding a liquid through suchdiffusion reactor downstream a nucleation valve is also anticipated.

The present invention according to one or more embodiments thereof mayprovide a deaerator system with increased energy efficiency

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a deaerator system according to a firstembodiment of the present invention.

FIG. 2 is a detail view of a portion of the deaerator system accordingto the first embodiment.

FIG. 3 is a graph illustrating the oxygen removal efficiency as afunction of the pressure drop for various temperatures relative to theflash boiling temperature.

DESCRIPTION OF EMBODIMENTS

Some portions of a system for processing a liquid will be describedreferring to FIG. 1. The present invention may form part of such asystem, though individual components may be replaced without departingfrom the scope of the invention as defined by the claims.

Starting at an upstream position, the system comprises a tank 2 or othersystem for holding or delivering the liquid to be processed. The systemalso comprises a pump 4 for increasing the pressure in the liquid,subjecting it to an elevated static pressure P_(UPSTREAM) such that itis forced downstream. The pump 4 may in one or more embodiments be acentrifugal pump, yet other alternatives may be feasible. Piping 6guides the liquid to the first processing step, namely to the nucleationvalve 8. Before describing details of the valve, some words about thearrangement downstream the valve should be mentioned. The piping 10guiding the liquid following the nucleation valve 8 debouches in aseparation vessel 12. In the present embodiment the separation vessel 12comprises an expansion vessel, connected to a vacuum pump 14 in an upperend, and connected to an evacuation system 16 for liquid in a lower end.Air and other gases resulting from the deaeration will be evacuated viathe upper end, while deaerated liquid will be pumped out via the lowerend of the vessel. To avoid evacuation of vaporized liquid the upper endof the vessel may comprise a condenser, condensing the vaporized liquidsuch that it may leave via the lower end instead. The vacuum pump 14generates a lowered static pressure pressure P_(DOWNSTREAM) propagatingto the downstream end of the nucleation valve 8.

Returning to the nucleation valve 8, the increased static pressureupstream the valve will push the liquid towards the nucleation valve 8and the lowered static pressure downstream the valve will pull theliquid, the relevant term to consider being the pressure drop over thevalve 8, which may be defined as ΔP=P_(UPSTREAM)−P_(DOWNSTREAM).

In the present embodiment P_(DOWNSTREAM) preferably corresponds to apressure above the vapor pressure at the residing temperature and forthe fluid being processed, such to avoid flashing, partly since thegeneration of flash is energy consuming. To this end it should bementioned that as the liquid passes the constriction of the valve itwill accelerate to a significant velocity, which may cause the dynamicpressure to momentarily drop below the vapor pressure.

The rapid pressure drop “shocks” the liquid such that a homogenousnucleation occurs, which is beneficial for deaeration. It has beenconfirmed in experiments that the momentary flash or cavitation in thevalve 8 is beneficial from a deaeration perspective. Immediately afterbeing homogenously nucleated the liquid enters the low pressure regiondownstream the valve in the form of a jet 18, which rapidly breaks upsuch that the liquid exposes a large surface area forming the interfacebetween gas and liquid. Conditions beneficial for deaeration thusprevail downstream the valve. This is schematically illustrated in FIG.2

The valve layout, e.g. in terms of exact shape of its nozzle orifice(s)is not crucial, yet some parameters seem to be important, and two areworth mentioning in particular: 1) The pressure drop should occurrapidly, basically instantly as the liquid passes the valve 8. Thisindicates that the valve construction should be non-complex, without anyintricate tubing following or preceding the nozzle orifice. 2) After thepressure drop the resulting jet should be non-constraint, i.e. a freejet should be allowed to form and break up. This also indicates that anon-complex valve construction without intricate tubing following theorifice is advantageous. In one embodiment the valve may be of a typehaving a conical regulating plug with a lip seal. This is a standardtype of valve and examples include the SPC-2 valve (Alfa Laval), whichis a sanitary electro-pneumatic regulating valve for use in stainlesssteel pipe systems. A simple hour-glass shaped restriction will alsodue, at least during constant operating conditions, yet a controllablevalve is preferred.

For the above reasons, a diffusion reactor 20 is arranged downstream thevalve 8, as part of the piping 10. The diffusion reactor 20 will enableturbulent diffusion of the dissolved gas in the liquid phase to the nowexisting and growing nuclei/gas bubbles, and it should have aconstruction not entailing a large pressure drop. In the embodiment ofFIG. 2 the diffusion reactor 20 comprises a rectilinear pipe, having adiameter such that it does not interact with the formation of thepreviously mentioned jet 18. Sooner or later as the jet 18 breaks up theflow will diverge and interact with the walls of the diffusion reactor,and even a non-breaking jet would sooner or later impact on the lowerwall due to gravity. The flow will continue towards the separationvessel, pulled by the vacuum, where it will be separated into a liquidflow and a gas flow. At some point the jet will fill the whole diameterof the diffusion reactor 20, the exact location depending on pressure,temperature, flow velocity, etc.

In the present embodiment the diffusion reactor 20 is arranged in ahorizontal direction. In a second embodiment the diffusion reactor maybe arranged in a vertical direction, with the jet coming from above.With this arrangement the pressure loss generated by the diffusionreactor will be compensated by the effect of gravity, reducing thelosses in the system. The diffusion reactor may be mounted in anyinclination between vertical and horizontal without departing from thescope of the present invention, as defined by the claims.

In the text below some operating parameters for embodiments of thepresent invention are listed, which may facilitate enablement for askilled person. The amount (or rate) of liquid being processed may be inthe order of up to about 100,000 I/h, though smaller flows are possible,and in experiments conducted flows in the order of 6,000 I/h have beenused. These rates are common within the field of the invention, anddetails in regard of pumps and such on the downstream side of the valve8 will not be discussed in detail.

The pressure drop over the valve ΔP preferably exceeds 2 bar, and it iseven more preferred that it exceeds 3 bar, and it may be as high as 4bar or 5 bar. There is no technical problem in increasing ΔP evenfurther yet the pump used to elevate the pressure will be increasinglyexpensive.

The temperature downstream the valve should preferably be lower than theflash temperature (the boiling point at the prevailing pressureP_(DOWNSTREAM)), such as −10° C. below flash or −5° C. or between thosetemperatures and the flash temperature. Temperatures closer to flashhave been found to increase the deaeration efficiency. Flash boilingwill still have a beneficial impact on the deaeration, yet experimentsverify that it is not as dramatic as for prior art systems.

The length of the diffusion reactor may be about 100-200 cm, yet it maybe even longer. A longer diffusion reactor will improve the deaerationefficiency, yet it may also increase pump losses, which is an unwantedfeature. The diameter of the diffusion reactor may be about 5 cm (2″pipe) and it may be manufactured from stainless steel. In theory thediameter of the diffusion reactor would benefit from being larger, sinceit would result in lower pressure loss, yet due to parameters related toworking at pressures close to vacuum may result in a tradeoff where thesuggested diameter is beneficial. Smaller diameters may result inreduced deaeration efficiency, supposedly due to a shorter hold up timeand an increased interaction between the jet (or spray) and the walls ofthe diffusion reactor, and due to increased pressure losses, e.g. makingthe pressure drop less abrupt.

There is no abrupt pressure drop as the liquid passes from the diffusionreactor 20 into the separation vessel 12, in which the separationprocess initiated in the nucleation valve 8 is finalized. The separationvessel 12 may therefore be of quite rudimentary design as compared toprior art systems where flashing takes place in the expansion vessel.Further, since flash boiling is avoided to a large extent, the amount ofvapor is reduced, resulting in that less energy has to be spent oncondensing the vapor.

All components of the system being in contact with the product should bemade from food grade material or approved for use when processingfoodstuff.

The pressure in the liquid upstream the nucleation valve, as well as theflow through the nucleation valve may be controlled by the nucleationvalve 8 and the pump 4, i.e. a frequency regulated pump, and for thesepurposes the pump 4 may also comprise a control valve (not shown).

If the temperature of the liquid upstream the nucleation valve 8 iscontrolled, this may be effected by means of a heat exchanger.

The pressure downstream the nucleation valve 8 is controlled by pressureregulation of the separation vessel 12.

The temperature of the liquid downstream the nucleation valve 8 isnormally not controlled in situations where no flash boiling occurs. Thepressure in the separation vessel 12 may be used to control thetemperature, if so desired.

In order to substantiate and validate the present invention according toseveral embodiments thereof extensive experimental studies wereconducted. In those experiments the flow of the liquid ranged from 3,000to 9,000 I/h, the relative flash temperature from −35 to +1° C.(negative indicating a temperature below flash boiling), and rangedbetween 1 and 5 bar. For each of the numerous measurement points severalaspects, such as oxygen concentration as a function of the positionafter the nucleation valve, the void fraction as a function of theposition after the nucleation valve, pressure as a function of theposition after the nucleation valve, the overall deaeration efficiency,the cavitation index, etc was measured, estimated or calculated. FIG. 3is a graph showing the oxygen removal efficiency as a function of ΔP forsome different temperatures (again the temperatures are given relativeto the flash temperature). The graph indicates that for a systemaccording to at least one embodiment of the present invention the oxygenremoval efficiency does not vary significantly between a temperatureslightly below the flash boiling temperature and a temperature slightlyabove the same.

1. A method for deaerating a liquid, comprising: pressurizing the liquidto a pressure above atmospheric, guiding the pressurized liquid througha nucleation valve, and lowering the pressure on a downstream side ofthe valve to a subatmospheric pressure, thereby causing bubblenucleation as liquid passes the nucleation valve as the first step ofthe deaeration, characterized in that the temperature and pressure onthe downstream side of the valve is controlled such that the staticpressure is above the saturation pressure, while the lowest pressure asthe liquid passes the valve is below or equal to the saturationpressure.
 2. The method of claim 1, further comprising a step ofinducing a pressure drop over the valve (ΔP), the pressure drop (ΔP)exceeding 2 bar.
 3. The method of claim 1, further comprising a step ofcontrolling the pressure immediately downstream the valve such that itremain below 150% of the saturation pressure for the liquid at thattemperature.
 4. The method of claim 1, further comprising a step ofguiding the liquid leaving the nucleation valve downstream directly intoa diffusion reactor, in which dissolved gas in the liquid will diffusefrom the liquid into the gas bubbles.
 5. The method of claim 1, furthercomprising a step of forming a free fluid jet after the nucleationvalve.
 6. The method of claim 4, further comprising a step of guidingthe liquid leaving the diffusion reactor downstream into a separationvessel.
 7. The method of claim 4, wherein at least 50% of the masstransfer from dissolved phase to gaseous phase occurs in the diffusionreactor. 8-17. (canceled)